Generalize list(seq) to work with iterators. This also generalizes list()

Generalize list(seq) to work with iterators.  This also generalizes list()
to no longer insist that len(seq) be defined.
NEEDS DOC CHANGES.
This is meant to be a model for how other functions of this ilk (max,
filter, etc) can be generalized similarly.  Feel encouraged to grab your
favorite and convert it!
Note some cute consequences:
    list(file) == file.readlines() == list(file.xreadlines())
    list(dict) == dict.keys()
    list(dict.iteritems()) = dict.items()
    list(xrange(i, j, k)) == range(i, j, k)